This application is related, generally and in various embodiments, to synchronous rectifier circuits.
Synchronous rectifier circuits for producing a unipolar DC voltage output from an alternating voltage input are well known and commonly used in a variety of power converter topologies such as, for example, half-bridge and full-bridge DC-DC power converters. Power converters having these bridge topologies typically include a bridge input circuit connected to a primary winding of an isolation transformer (“primary stage”), and a synchronous rectification circuit connected to a secondary winding of the isolation transformer (“secondary stage”). The bridge input circuit typically includes switching devices (“primary switches”), such as field effect transistors (FETs), for converting a DC input voltage into an alternating voltage. The alternating voltage is coupled from the primary winding to the secondary winding and then rectified by the synchronous rectifier circuit to produce a DC output voltage. Increasing or decreasing the duty cycle of the primary switches using, for example, pulse-width modulated (PWM) control signals, produces a corresponding increase or decrease in the DC output voltage.
Rectification of the alternating voltage is typically performed using metal-oxide-semiconductor field effect transistor synchronous rectifiers (MOSFET SRs). Because the forward voltage drop across a MOSFET SR, and hence its power loss, is less than that of a diode, MOSFET SRs are more efficient than diode-based rectifiers, particularly for low output voltage applications. Unlike a diode, however, MOSFET SRs may conduct current in both directions (i.e., forward and reverse). Accordingly, a synchronous rectifier circuit typically includes a gate drive circuit for supplying a gate control signal to a gate terminal of each MOSFET SR in order to render it non-conductive during reverse bias. Depending upon the configuration of the gate drive circuit, the synchronous rectifier circuit may be classified as “control-driven” or “self-driven.” In a control-driven scheme, the gate control signals are generated indirectly by a separate gate drive circuit. The gate drive circuit may be controlled, for example, using a gate drive transformer driven by the PWM control signals of the primary stage. In a self-driven scheme, the gate drive circuit utilizes existing power signals to control the MOSFET SRs. For example, the gate control signals may be obtained from an auxiliary secondary winding, or directly from the secondary winding of the isolation transformer.
In self-driven synchronous rectifier circuits having an auxiliary secondary winding as described above, it is known that MOSFET SR switching efficiency may be improved by introducing a positive voltage shift to the gate drive signals using gate rectification diodes. During periods in which the voltage across the auxiliary secondary winding is zero (i.e., during “deadtime”), the shifted gate drive signals render the MOSFET SRs conductive. Thus, “freewheeling” current that would otherwise flow through a lossy MOSFET SR body diodes instead flows through the MOSFET SR channels, resulting in decreased power loss.
Use of gate rectification diodes may have deleterious consequences, however, if the power converter is turned off (i.e., the primary switches are turned off) during the flow of negative inductor current in the synchronous rectifier circuit. Negative current flow may occur, for example, during a period after the power converter is turned on if the power converter is configured in parallel with other operating power converters. Negative current may also occur during operation if the duty cycle of the primary switches is decreased in order to command a lower output voltage. If the power converter is turned off under such circumstances, the dissipation of negative inductor current flow in the synchronous rectifier circuit will induce current flow in the auxiliary secondary winding and the primary winding. The MOSFET SRs will thus continue to switch, causing the synchronous rectifier circuit to self-oscillate at a frequency determined by the time required for the MOSFET SRs to discharge the shifted gate voltages introduced by the gate rectification diodes. Because the negative inductor current flow in the synchronous rectifier circuit progressively increases during each switching cycle of the MOSFET SRs, damaging voltages may be applied to the MOSFET SR gates, the bridge input circuit, and other power converter components.
In control-driven synchronous rectifier circuits, similar consequences may result if the MOSFET SRs are turned off during the flow of negative inductor current. In particular, although turning the MOSFET SRs off effectively prevents self-oscillation in the synchronous rectifier circuit, the paths necessary for dissipating negative inductor current flow are eliminated. Damaging voltage spikes resulting from uncontrolled inductor discharge may thus occur.
Accordingly, there exists a need in self-driven and control-driven synchronous rectifier circuits for a manner to controllably dissipate negative inductor current flow when the power converter is turned off.